Changes of air temperature and air pressure cause air temperature and air pressure-dependent fluctuations of the air/fuel ratio λ to occur. Combustion devices are therefore typically set with an air surplus to ensure sufficient oxygen for full combustion. This measure serves to avoid unhygienic combustion. The disadvantage of setting combustion devices to an air surplus is a lower level of efficiency of the system.
Most systems use rotational speed sensors and air pressure switches for measuring the amount of air. Rotational speed sensors are not entirely sensitive to fluctuations in air temperature and air pressure. Air pressure switches are precise at a specific pressure. Given this, many systems include a number of switches to monitor air pressure at that number of pressures. Despite this, adjusting the entire operating range of the combustion device is as yet barely possible.
The occurrence of turbulence makes the problem even more difficult, since the signal of a flow sensor is greatly influenced by its installed position in the middle of a turbulent flow. As well as this, the turbulence causes the measurement signal to be very noisy.
European patent EP1236957B1 describes the adaptation of a burner-operated heating device to an air exhaust system using a pressure sensor/air mass sensor 28, which is arranged in the air feed 14 or exhaust gas venting system of a heating device. A regulating device 30 regulates a fan 26, starting from the signal of the sensor 28. To adapt the instantaneous air volume flow to a required air volume flow, an operating characteristic curve 40 is stored. To improve the regulation behavior with large differences in temperature and with respect to emergency operating characteristics a temperature sensor 35 is provided.
European patent EP2556303B1 describes a pneumatic composite having mass balancing with a venturi nozzle 5, which creates a vacuum, with a mass flow sensor 6 in an additional duct 7. An open-loop or closed-loop controller 9 regulates the speed of a fan 1 as a function of the signal of the sensor 6.